Method and apparatus for achieving gain in electron radiography imaging chamber

ABSTRACT

An electron radiography system with improved resolution and high gain. An electron radiography system with a few percent of an electronegative gas added to the imaging gas and operable for short intervals in the avalanche, high gain condition. Arrangements for obtaining periodic high gain operation including gap voltage increases and gap radiation.

United States Patent [191 Proudian METHOD AND APPARATUS FOR ACHIEVING GAIN IN ELECTRON RADIOGRAPHY IMAGING CHAMBER [56] References Cited UNITED STATES PATENTS [111 3,835,321 Sept. 10, 1974 3,774,029 11/1973 Muntz .L 250/315A Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or Firm-Harris, Kern, Wallen & Tinsley ABSTRACT An electron radiography system with improved resolution and high gain. An electron radiography system with a few percent of an electronegative gas added to the imaging gas and operable for short intervals in the avalanche, high gain condition. Arrangements for obtaining periodic high gain operation including gap voltage increases and gap radiation.

g 10 Claims, 2 Drawing Figures 3,652,852 3/1972 Miyazawa..; 250/315 A 55 1 f6 .11 uy p 1 .17 jg Pan E2 g 9 l %ggg ufil s43- mcw/ VL I g@ V0L7'A76E V Jl/PpLi H 14 i5 arc/M4704 PATENTED SEP 1 01974 L 0MP pa we e Sl/PFL l l l I x0 10 v, HPPL lED VOL 746E METHOD AND APPARATUS FOR ACHIEVING GAIN IN ELECTRON RADIOGRAPHY IMAGING CHAMBER This invention relates to electron radiography and in particular, to an improvement on the high resolution system disclosed in the copending application of Proudian et al. entitled IMAGING GAS FOR IM- PROVED RESOLUTION IN IMAGING CHAMBER OF ELECTRON RADIOGRAPHY SYSTEM, filed concurrently with this application and assigned to the same assignee.

In electron radiography, an X-ray opaque gas is positioned in a gap between electrodes of an imaging chamber, with incoming X-rays being absorbed in the gas and producing electrons and positive ions. The electrons may be collected on a dielectric sheet placed on the anode, resulting in a latent electrostatic image on a sheet which is then made visible by xerographic techniques.

As discussed in said copending application, one of the limits of resolution in the electron radiographic systern of X-ray imaging, is the charge diffusion limit,

whereby charges created by an absorbed X-ray photon form a spot of finite size by the time they are collected at the latent image receptor, due to the diffusive (random thermal) motions of the charge carriers, be they ions or electrons.

The aforesaid copending application discloses an effective means for minimizing diffusion effects on resolution, namely, the use of a heavy negative ion as charge carrier, which ion may be created by the addition of a few per cent of a strongly attaching electrophyllic gas such as SF to the X-ray opaque gas in the imaging chamber. The electronegative gas rapidly captures the electrons created by the primary photoelectrons released by the absorbing radiopaque gas, e.g., Xenon, thereby inhibiting the image blurring due to the highly diffusive electrons. The negative ions formed thereby, e.g., SF do not readily recombine with the positive ions created during the ion pair formation process, so that a full signal is carried by the negative ions.

By its very nature, the above method for improving resolution no longer permits operation of the chamber at other than unity gain, since the collision processes between electrons and neutrals which lead to avalanche multiplication in the gas (and which would result in diffusive widths controlled byelectron transport) are specifically inhibited by capture of the free electrons, so that they cannot pick up energy from the field and induce further ionizations.

On the other hand, it is desirable, in order to maximize the sensitivity of the electron radiographic process, to maximize the charge collected on the latent image receptor, since there is a lower limit to the charge which can effectively be developed, in a reasonable time and with reasonably practical apparatus, by xerographic techniques.

The problem thus presented is to produce an effective gain in the imaging chamber greater than unity, without at the same time seriously degrading the image resolution due to electron transport processes. It is an object of the present invention to provide new and improved methods and apparatus for achieving these two seemingly conflicting results.

The solution to the problem consists of operating the imaging chamber in a hybrid mode, i.e., sequentially in a high gain condition and in the unity gain condition.

As set out in said copending application, the lateral diffusion of charges which drift a distance (I under the influence of an applied potential difference V(d) over the distance d has a characteristic length scale and the low diffusivity of heavy ions in the imaging chamber is due to the fact that the thermal energy E of the heavy ions is very small (equal to room temperature neutral atom energy 1/40 electron volt), whereas that of electrons, under the influence of a strong field, approaches the ionization energy value of the most easily ionized gas component, around 10 ev or so for Xenon.

The lateral diffusion, even that due to electrons, over a sufficiently small drift distance d, will be small, as indicated by Equation 1. Thus, the present method for obtaining gain as well as high resolution is to operate the chamber at high gain only over a very small portion of the gas gap, or more specifically over a very small portion of the total travel of the charges from their point of creation to their point of collection.

In the presently preferred mode of operation, the chamber is operated in the high gain mode for short periods t occurring at intervals T, and in the unity gain mode between the periods I, such that the total fractional duration of the high gain mode,

is much less than unity, typically about l/lO.

In order to achieve high gain operation of the chamber during the desired time periods I, electron avalanches are produced during the periods. These may be achieved by providing a high voltage (AC or DC) across the electrode gap during the periods t. Bursts of visible or ultraviolet radiation in an appropriate wavelength band and at the same intervals T, and of duration t I, may be used in conjunction with the increased voltage. It is also possible to use only the bursts of radiation during t at intervals T, without the high voltage pulses. All three modes of operation, voltage pulses alone, voltage plus radiation pulses, and radiation pulses alone, can be explained in terms of gain charac teristics for the chamber gas, including the electronegative gas additive, both in the presence and the absence of the radiation.

Other objects, advantages, features and results will more fully appear in the course of the following description. The drawing merely shows and the description merely describes preferred embodiments of the present invention which are given by way of illustration or example.

In the drawing:

FIG. 1 illustrates an electron radiographic system with imaging chamber and incorporating the presently preferred embodiment of the invention; and

FIG. 2 is a diagram illustrating the operation of the system of FIG. 1.

X-rays are directed from a source past the object 11 being X-rayed to the imaging chamber 12, which may be conventional in design. A typical imaging chamber includes a housing 13 carrying a cathode 14 on an insulator 15, with an anode 16 carried on the housing cover 17. In an alternative configuration, the housing cover can serve as the anode. The dielectric sheet receptor 18 may be carried on the anode, with the gas introduced into the chamber at 19 filling the gap between the electrodes. The gas is a high Z, radiopaque gas, typically Xenon or Krypton, maintained at a high pressure in the gap, typically 20 atmospheres. A small amount of a quenching gas such as methane may be included. Also included is a few per cent of an electronegative gas, as described in said copending application.

An electric field is provided across the gap between the electrodes from a high voltage supply which provides a lower output voltage V at terminal 26 and a higher output voltage V at terminal 27. In the embodiment illustrated, the high voltage supply provides a negative voltage which is connected to the cathode 14 through switch 30, with the anode 16 connected to circuit ground.

A source of radiation, typically a tube lamp 34, is mounted in the chamber for directing radiation into the gap between the electrodes. The lamp is energized from a lamp power supply 35 via a switch 36.

The switches 30, 36 may be contacts of a relay actuated by a relay coil 37 driven by an oscillator 38 for switching between the non-avalanche, unity gain operating condition'with the lamp off and the lower voltage V,, connected across the electrodes, and the avalanche, high gain condition with the lamp on and the higher voltage V connected across the electrodes.

The effect of the electrophyllic gas additive is to capture free electrons in the gas, forming a negative ion. The electrons, once attached, are not readily detached by collision, and can no longer absorb energy from ambient fields. Thus, to initiate an avalanche, one can either make use of residual free electrons, if they survive in sufficient numbers, or one may redetach the electrons bound to the electrophyllic molecules, and this is one of the purposes of the radiation pulse. Note that the radiation must be capable of producing photo detachment from the negative ions, but not be capable of ionizing neutral species, since this would lead to collection of spurious current.

In other words, the radiation frequency v should lie in the band where E is the photo detachment threshold, typically one to four electron volts for the attaching species of interest, E, is the photo ionization threshold for the most readily ionizable neutral species in the gas, typically over 10 electron volts, and h is Plancks constant. Typically the radiation from the lamp 34 is in the range of 2 to 5 electron volts, corresponding to the visible to near ultraviolet portion of the electromagnetic spectrum. Many sources of radiation, and in particular are discharge and spark discharge lamps, are available in this wavelength region, and quartz windows readily transmit those wavelengths, so that no difficulties arise in producing the required radiation.

The three possible modes of chamber operation are described in conjunction with FIG. 2, which depicts approximately the first Townsend coefficient a as a func-- dildx=ai where dx is an elementary displacement inthe direction of the field, for a DC field.

For an AC field, an equivalent coefficient exists, with the current or more properly the growth of free electron density n as a function of time t, is governed by d n /d t an;

The gain curve 40 for a in the presence of radiation has been taken as the same as would obtain in the absence of the electronegative gas additive, since the effect of the radiation is specifically to neutralize, literally as well as figuratively, the additive, by countering the attachment by photo detachment. The gain curve 41 is for the presence of the attacher additive without radiation. I

The curves in FIG. 2 represent the following situation: in operation at voltage V typically 10 to 15 kilo volts, the coefficient a is zero, so that the chamber operates at unity gain, with or without the additive, i.e., with or without radiation.

In operation at voltage V typically 30 to kilovolts, and in the absence of radiation (i.e., when the attaching additive is effective), the gain coefficient a is still zero, so that the chamber operates at unity gain. However, when the radiation is on (so that the additive is rendered ineffective), then the coefficient a is positive, say a 10 (in units of inverse gap width), so that the gain, for steady state operation of the chamber in the high gain mode will be and for operation in that mode for a fraction f of the time will be providing that the pulse t and the interpulse interval T will satisfy the conditions:

t electron transit time across gap T ion transit time across gap (7) In operation at voltage V typically to I00 kilovolts, the coefficient will be positive, say again a 10, even in the absence of radiation, or in other words the electron avalanche will proceed in spite of the attaching additive (the dielectric strength of the gas with additive will have been exceeded).

The imaging chamber operating condition options are the following: (i) Operate the chamber with steady state applied voltage V,, and apply voltagepulses of value V and simultaneous radiation pulses sufficient to suppress the additive, at intervals T and for duration t. (ii) Operate the chamber with steady state voltage V,, and apply only voltage pulses of value V (iii) Same as (ii), but use an initiating pulse of radiation of duration t t. (iv) Operate the chamber at steady state voltage V and apply only radiation pulses. While uniform periods t at regular intervals Tare illustrated and preferred, they are not essential. However the periods should be less than the transit time of electrons across the gap and the intervals should be less than the transit time of the ions across the gap.

The power required to photodetach substantially all electrons bound to the electronegative molecules can be readily estimated. Since typically, the number of charges collected on the receptor is an average of charges/cm and the exposure time is normally 10 sec, there are 10 charges/cm -sec collected or, for an image area of 2,000 cm (14 inches X 17 inches), there are 2 X 10 charges collected per second. The average transit time of the charges across the gap is about 5 X 10 sec, so that the number N of charges present in the chamber at any one time is around 10 If the charges are converted from ions to free electrons, the latter are recaptured in a time L lO seconds. The power P required to maintain detachments is then P N (E /ya), E detachment energy (8) and using N 10", E -3 electron volts==5 l0 Joules, ya 10 sec, we find P z 5 watts Thus, even assuming an overall efficiency of the radiation system (lamp electrical conversion efficiency, fraction of radiation in useful frequency range, radiation losses in chamber, optics collection efficiency) of only 0.01 percent, which is quite conservative, the actual input power required is only ,5 X 10 watts, or about a quarter of a Joule in a 5 a sec pulse. This is very easily attainable. 1

A pulse duration t -5 1 sec would be a typical operating value. For a voltage pulse V=5 X 10 volts, also a typical value, the voltage switching requirement would be of the order 2Vlt-2 X 10" volts/sec, also an easily attainable requirement, for either AC or DC fields. A typical value for the interpulse interval is 5 X 10 seconds (about one-tenth the ion transit time), and a typical value for f would be 0.1. The electron diffusion in a period of 5 .t sec would be of the order of 3 X 10 cm, so that for 10 pulses per transit time,

D -3 X 10 X 10 -10 cm, which is excellent resolution (approximately 10 line pairs per mm).

In operation, the dielectric sheet 18 is mounted on the anode and the imaging chamber is sealed. The gas is introduced into the gap at the appropriate pressure and the high voltage supply is turned on. The oscillator is energized to switch between the unity gain and high gain conditions, the x-ray source 10 is energized for the desired exposure, and the electrostatic charge image is formed on the dielectric receptor. The power supply is then shut off, the chamber is exhausted to atmospheric pressure and opened, and the dielectric sheet is removed for subsequent developing and fixing.

If both radiation and increased potential are used to achieve the increased gain, both switches 30 and 36 are utilized. If only radiation is used, switch 30 is omitted with the cathode connected directly to terminal 26. If only the potential increase is used, the lamp and power supply are omitted. If the radiation is to be turned on only during the initial portion of the time interval t, a timing circuit such as a simple rc circuit can be used in the lamp power supply for energizing the lamp.

I claim: 1. A method of producing an electrostatic image on a dielectric sheet, including the steps of:

positioning the dielectric sheet at an electrode in a gap between anode and cathode electrodes positioned adjacent an object to be imaged; passing X-rays through saidobject and one of said electrodes;

absorbing incoming X-rays in the gap by maintaining in the gap an X-ray opaque gas of atomic number at least 36 at superatmospheric pressure and generating electrons and positive ions in the gas; converting electrons in the gap to negative ions by combining electrons with electronegative gas molecules maintained in the gap with said X-ray opaque attracting negatively charged particles toward the anode by applying a gap potential across the electrodes'depositing said charged particles on the dielectric sheet, said gap potential being of a value such that the chamber is operating in the nonavalanche, substantially unity gain condition; and

repeatedly changing the chamber to the avalanche, high gain condition with the chamber operated in the high gain condition for time periods short relative to the transit time of electrons across the gap, with the intervals of unity gain between the high gain periods being short relative to the transit time of the negative ions across the gap.

2; The method of claim 1 including changing the chambers to the high gain condition by increasing the gap potential.

3. The method of claim 1 including changing the chamber to the high gain condition by directing into the gap, radiation of a frequency producing photo detachment of electrons from the negative ions.

4. The method of claim 3 wherein the radiation is in the visible to near ultraviolet portion of the spectrum.

5. The method of claim 1 including changing the chamberto the high gain condition by increasing the gap potential and by directing into the gap, radiation of a frequency producing photodetachment of electrons from the negative ions.

6. The method of claim 5 with the potential increased for a time period short relative to the transit time of electrons across the gap, and with the radiation directed into the gap for the initial portion of each of the time periods.

7. In a radiographic system for operation with a source of X-rays and having a pair of electrodes comprising an anode and a cathode,

first means for supporting said electrodes in spaced relation with a small gap therebetween and for maintaining a superatmospheric pressure in said gap with a dielectric sheet in said gap at said anode,

an X-ray absorber and electron and positive ion emitter in said gap between said anode and cathode for producing a charge image on said dielectric sheet,

said emitter comprising an X-ray opaque gas at superatmospheric pressure and having an atomic number of at least 36,

second means for connecting an electric power supply across said electrodes of a potential for operating in the non-avalanche, substantially unity gain condition in said gap, attracting negatively charged particles toward said anode for deposit of said charged particles on said dielectric sheet,

with said X-ray opaque gas including a few per cent of an electronegative gas for combining with electrons in said gap forming negative ions for attraction to said anode, and

third means for changing the operating condition in said gap to the avalanche, high gain condition wherein said third means includes means for switching between the unity and high gain conditions, maintaining the high gain conditions for time periods short relative to the transit time of electrons across the gap and maintaining the intervals of unity gain between the high gain periods short relative to the transit time of the negative ions across the gap.

8. A system as defined in claim 7 wherein said third means includes means for increasing the potential connected across said electrodes.

9. A system as defined in claim 8 wherein said third means includes a radiation source for directing into said gap, radiation of a frequency producing photodetachment of electrons from the negative ion.

:10. A system as defined in claim 7 wherein said third means includes a radiation source for directing into said gap, radiation of a frequency producing photodetachment of electrons from the negative ion.

UNITED STATE .5 PA'i'i-T OFFICE Y CER'II-FICIAJIE OF CORRECTION Patent No. 3,835,321- Dated September 10, 1974 Invent0r(s) Andrew P. Proudian It is certified that error appears the above-identified patent and that said Letters Patent is hereby corrected as shown below:

Column 2, line 47 "t' r, should be t' 5 r,

Column 5, line 52, equation "D .2: 3 x 10' x- I 10 10" c-m, should be:

Signed and sealed this 3rd day of December 1974.

(SEAL) Attest:

McCOY 4. GIBSON JR." c. MARSHALL 'DANN Attestlng Officer Commissioner of Patents 

1. A method of producing an electrostatic image on a dielectric sheet, including the steps of: positioning the dielectric sheet at an electrode in a gap between anode and cathode electrodes positioned adjacent an object to be imaged; passing X-rays through said object and one of said electrodes; absorbing incoming X-rays in the gap by maintaining in the gap an X-ray opaque gas of atomic number at least 36 at superatmospheric pressure and generating electrons and positive ions in the gas; converting electrons in the gap to negative ions by combining electrons with electronegative gas molecules maintained in the gap with said X-ray opaque gas; attracting negatively charged particles toward the anode by applying a gap potential across the electrodes depositing said charged particles on the dielectric sheet, said gap potential being of a value such that the chamber is operating in the nonavalanche, substantially unity gain condition; and repeatedly changing the chamber to the avalanche, high gain condition with the chamber operated in the high gain condition for time periods short relative to the transit time of electrons across the gap, with the intervals of unity gain between the high gain periods being short relative to the transit time of the negative ions across the gap.
 2. The method of claim 1 including changing the chambers to the high gain condition by increasing the gap potential.
 3. The method of claim 1 including changing the chamber to the high gain condition by directing into the gap, radiation of a frequency producing photo detachment of electrons from the negative ions.
 4. The method of claim 3 wherein the radiation is in the visible to near ultraviolet portion of the spectrum.
 5. The method of claim 1 including changing the chamber to the high gain condition by increasing the gap potential and by directing into the gap, radiation of a frequency producing photodetachment of electrons from the negative ions.
 6. The method of claim 5 with the potential increased for a time period short relative to the transit time of electrons across the gap, and with the radiation directed into the gap for the initial portion of each of the time periods.
 7. In a radiographic system for operation with a source of X-rays and having a pair of electrodes comprising an anode and a cathode, first means for supporting said electrodes in spaced relation with a small gap therebetween and for maintaining a superatmospheric pressure in said gap with a dielectric sheet in said gap at said anode, an X-ray absorber and electron and positive ion emitter in said gap between said anode and cathode for producing a charge image on said dielectric sheet, said emitter comprising an X-ray opaque gas at superatmospheric pressure and having an atomic number of at least 36, second means for connecting an electric power supply across said electrodes of a potential for operating in the non-avalanche, substantially unity gain condition in said gap, attracting negatively charged particles toward said anode for deposit of said charged particles on said dielectric sheet, with said X-ray opaque gas including a few per cent of an electronegative gas for combining with electrons in said gap forming negative ions for attraction to said anode, and third means for changing the operating condition in said gap to the avalanche, high gain condition wherein said third means includes means for switching between the unity and high gain conditions, maintaining the high gain conditions for time periods short relative to the transit time of electrons across the gap and maintaining the intervals of unity gain between the high gain periods short relative to the transit time of the negative ions across the gap.
 8. A system as defined in claim 7 wherein said third means includes means for increasing the potential connected across said electrodes.
 9. A system as defined in claim 8 wherein said third means includes a radiation source for directing into said gap, radiation of a frequency producing photodetachment of electrons from the negative ion.
 10. A system as defined in claim 7 wherein said third means includes a radiation source for directing into said gap, radiation of a frequency producing photodetachment of electrons from the negative ion. 